Stage atmospheric distillation unit

During the past several years such stills have been largely replaced by tube or pipe stills because of their lower initial cost, greater throughput, and economy of operation. A common type of operation utilizes a two-stage atmospheric and vacuum distillation unit (13). This type of operation has an important advantage—the asphaltic residue remains at the extreme temperature for only a fraction of a minute in the pipe stills as contrasted to several hours in shell-type stills. 

In two stage units, it is often economical to distill more gas oil in the vacuum stage and less in the atmospheric stage than the maximum attainable. Gas formed in the atmospheric tower bottoms piping at high temperatures tends to overload the vacuum system and thereby to reduce the capacity of the vacuum tower. The volume of crude vaporized at the flash zone is approximately equal to the total volume of distillate products. Of course, the vapor at this point contains some undesirable heavy material and the liquid still contains some valuable distillate products. The concentration of heavy ends in the vapor is reduced by contact with liquid on the trays as the vapor passes up the tower. This liquid reflux is induced by removing heat farther up in the tower. 

In two stage units, it is often economical to distill more gas oil in the vacuum stage and less in the atmospheric stage than the maximum attainable. 

In view of these considerations, a large amount of effort is reported in the scientific press on the development of a process to produce benzene from n-hexane by combined cyclization and dehydrogenation. w-Hexane has a low Research octane number of only 24.8 and can be separated in fair purities from virgin naphthas by simple distillation. Recently, an announcement was made of a process in the laboratory stage for aromatiza-tion of n-hexane (16). The process utilizes a chromia-alumina catalyst at 900° F., atmospheric pressure, and a liquid space velocity of about one volume of liquid per volume of catalyst per hour. The liquid product contains about 36% benzene with 64% of hexane plus olefin. The catalyst was shown to be regenerable with a mixture of air and nitrogen. The tests were made on a unit of the fixed-bed type, but it was indicated that the fluid technique probably could be used. If commercial application of this or similar processes can be achieved economically, it could be of immense help in relieving the benzene short-age. 

A liquid containing 60 mole% toluene and 40 mole% benzene is continuously fed to and distilled in a single-stage unit at atmospheric pressure. What percent of benzene in the feed leaves in the vapor if 90% of the toluene entering in the feed leaves in the liquid Assume a relative volatility of 2.3 and obtain the solution graphically. 

Sharing of past major incidents with other oil and gas industries provides useful input data for similar process industries in order to identify the most critical barriers and improve their safety processes. One poignant example highlights this matter. In 1998 there was an accident in the gas compression stage of a Middle East oil and gas plant which caused 7 dead as a result of fuel accumulation and vapor cloud explosion which was very similar to the Texas City Refinery disaster on March 23, 2005 in which a distillation tower was overfilled and an uncontrolled release of hydrocarbons led to a major explosion and fires. Fifteen people were killed and 180 were injured in the worst disaster in the United States in a decade. In both incidents, excess hydrocarbons were diverted into a pressure relief system that included a blowdown stack. In the Iranian case, it was equipped with a flare, but one which the operator didn t ignite in Texas City the blowdown stack was not equipped with a flare to burn off hydrocarbons as they were released. As a result, the flammable overflow from the tower entered the atmosphere. Ignition of the escaped hydrocarbons was enabled by startup of a nearby vehicle resulted in the explosion and subsequent fires (Hopkins, 2008). This example shows the repetitive patterns of accidents, and root causes of events all over the world in this sector. The lesson of this paper is that accidents in one country, where the scenarios are very similar, can and should serve as lessons to prevent the same scenario being actualized in other countries. 

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